The present invention relates to systems for analyzing rotating recording discs. More specifically, the present invention relates to glide heads for analyzing the surface of a rotating recording disc.
In data processing systems, magnetic disc drives are often used as data storage devices. In such devices, read/write heads are used to write data on or read data from a rotating hard or flexible disc. When writing information to the disc, the write head generates a magnetic field that changes the magnetic moment of a small localized area on the disc. The size of the small localized area partially determines the data density for the disc. In general, write heads that fly closer to the disc create smaller localized areas and as such increase the amount of data stored on the disc.
In order to minimize fly height and thus maximize data density, discs must be produced under strict tolerances. For example, a disc must be free of defects or "asperities" that extend above the general surface of the disc; the data zones of the disc must have an average roughness or irregularity in the surface that meets certain fixed guidelines; and, the landing zone in the disc, the area of the disc where the head comes to rest when the disc is not spinning, must have a sufficient roughness so that the head does not stick to the surface. If the landing zone is too smooth, the smooth surface of the head will partially bond to the landing zone making it difficult to lift the head when the disc begins spinning.
To determine the average roughness of the disc both inside and outside of the landing zone, a glide head is flown over the disc to perform a test known as glide avalanche. The speed of the disc is reduced until the glide head begins to make nearly constant contact with the disc. This contact is determined by a sensor that is embedded on the glide head. In some cases, the sensor is a piezoelectric device that generates an electrical signal when it is mechanically deformed. In other instances, the sensor is a magneto-resistive head element, which experiences a change in its resistance due to the heat generated by the contact between the glide head and the surface. Based on the electrical signals provided by either type of sensor, it is possible to determine when the glide head experiences nearly constant interference from the surface of the disc. At this point, the height of the glide head is measured using a laser interferometer. This glide height then provides a measure of the roughness of the disc both within the landing zone and within the data zone of the disc.
To determine the number of defects on the disc and their location, the glide head is generally flown a fixed distance above the disc, for example one microinch. At this height, the glide head flies freely over the surface of the disc and only experiences periodic interference from asperities or defects on the disc. When the glide head impacts an asperity, the sensor conveys the impact through an electrical signal, and the asperity is thus recorded.
In the test for determining the roughness of the disc and the test for detecting asperities on the disc, it is preferred that the glide head have a very sharp and even impact edge. The impact edge is the portion of the glide head that is closest to the surface as the glide head flies over the disc. As such, the tests measure interference between the impact edge and the surface of the disc. By having a very even edge, there is less likelihood that an individual asperity will pass through a gap in the edge. In addition, an even and sharp edge maximizes the likelihood that the entire edge comes into contact with the surface during glide avalanche testing. This improves the determination of average roughness by providing a sharper interference signal.
Although it is desirable to have a sharp and even impact edge for testing, the tests tend to destroy this edge. Specifically, each of the tests requires that the impact edge of the glide head come into contact with the surface. As a result, the impact edge experiences a large number of impacts with the surface that cause damage to the edge such as liftouts, where large chunks of edge material are removed from the edge. After a period of testing, the impact edge becomes so damaged, that it can no longer be used to determine the characteristics of a disc surface. At this point, the glide head must be replaced.
The susceptibility of the impact edge to damage is increased by the fact that prior art impact edges are formed through abrasive techniques such as diamond grinding. These abrasive techniques tend to introduce a large amount of stress into the areas of the glide head surrounding the impact edge. These grinding techniques are used because they do not require the large capital investment associated with other techniques that have been used to shape advanced read/write heads. In particular, diamond grinding is much less expensive than photolithography techniques, which require a "clean room".
The impact edges of existing glide heads are also susceptible to damage because of the materials they are made from. In particular, existing glide heads are constructed from a composite material of alumina oxide with titanium carbide (AL.sub.2 O.sub.3 TiC). Such multi-phase materials are susceptible to non-uniform wear and chipping.